In the field of clinical diagnostics, there exists a need for reliable, low cost devices, i.e., for the routine quantitative determination of electrolytes and certain metabolites in physiological fluids. For this purpose, ion selective electrodes (ISE) have routinely been applied in the clinical measurement of sodium, potassium, chloride and similar ions. Most ion selective devices on the market today have a limited lifetime in the sense that the materials of construction tend to deteriorate with use and time. For example, flow conduits become coated and clogged due to clotting, or coagulation, of protein. Also the ion sensing membranes tend to degrade in performance with time due to membrane poisoning by sample constituents or general loss of ionophore potency. Such electrodes typically require a high degree of maintenance. It is desirable, therefore, to be able to replace the ion selective electrodes simply and conveniently with a low cost assembly before performance deteriorates when the occasion or need arises. This is preferable to the substantial maintenance that would be otherwise required to achieve high operating efficiency and effectiveness.
Disposable solid state sensors possess significant advantages over ion selective electrodes of conventional design. The ability to discard the sensor after each or several uses is highly desirable. A number of the sensor designs described in the prior art have sought to achieve this end result.
U.S. Pat. No. 4,994,167, issued to Schults et al., discloses a device in which a removable cartridge protects a membrane of an electrode assembly. However, the design does not define flow channels and also does not provide a particularly good seal, thus the unit is incapable of being used for analyzing multiple ions.
Another United States patent, issued to Pfab et al , U.S. Pat. No. 5,018,527, describes a complex flow system having multiple solid state membrane sensors, but the sensors are complex and difficult to manufacture.
In United States patent issued to Calzi, U.S. Pat. No. 4,966,670, an apparatus is provided for the differential measurement of electrolytes using a conventional ISE design. Unfortunately, the apparatus described uses bored channels disposed within a relatively large mounting block. This makes the device relatively expensive and requires relatively large volumes of sample and calibration solutions to operate. The need of present day sensors is that they efficiently operate with relatively small reagent volumes and they should be of low cost so that it is feasible to make them easily and they are readily replaceable. U.S. Pat. No. 4,871,439 issued to Enzer et al. incorporates a plurality of solid state electrodes in a disposable cartridge. The cartridge includes calibration solutions in self contained containers, a reference electrode fluid container and a container for spent fluids. These subassemblies are all integrated into a single structure. The whole unit is disposable but represents a relatively expensive, complex unit that is not suitable for the low cost testing large numbers of samples.
Finally, U.S. Pat. No. 4,786,394 Enzer et al. disclose a clinical analyzer which comprises a disposable cartridge containing sensors and further describes an electrically connected plug-in contained within the disposable cartridge, described previously, which contains bags for reagents, calibrators, waste dispose, etc.